1. Field of the Invention
The invention relates to a connection between the sections of a carbon or graphite electrode which sections are each provided at the end face with a conical threaded socket. The connection is a double-conical threaded nipple screwed into the sockets.
2. Description of the Prior Art
The carbon and graphite electrodes used in the manufacture of electric-furnace steel, steel alloys, silicon and the like, called electrodes in the following, consist generally of several cylindrical parts or sections screwed to each other. The sections are provided at the end face with recesses or sockets, into which a screw thread is worked. Double-conical threaded nipples which likewise consist of carbon or graphite are screwed into the sockets in order to mechanically and electrically connect the adjoining electrode sections to each other.
In the operation of the electrode in arc furnaces, the joint between the electrode sections and the nipple is subjected to mechanical stresses which are produced primarily by temperature gradients at the joint. The mechanical stresses are obviously a minimum if the threaded nipple is inserted into both sockets to the same depth, i.e. if the thread clearance is nearly constant over the entire length of the thread. In case of an asymmetrical screw connection, the induced mechanical stress can trigger the breakage of the connection. Accordingly, numerous proposals for making a symmetrical nipple connection between the sections of an electrode have been suggested. According to German Published Prosecuted Application No. DE-AS 1 029 105, the nipple is screwed into the socket of the first section up to the stop. Then the nipple is turned back to a slight extent but sufficient that after screwing-on the second electrode section, the threads of both nipple halves make contact with the thread flanks of the counterthread only when the flanks face the connecting surface. Since, under rough operating conditions, the amount of reverse screwing cannot always be observed with sufficient reliability, it has been proposed, for making a symmetrical nipple connection, to limit the screwing-in depth of the nipple into the socket of the first section by a stop perhaps in the form of a deformable spacer (for instance, German Pat. No. 10 61 458, German Pat. No. 1 272 472). The spacers are inserted here into recesses of the socket bottom. However, a symmetrical seat of the nipple is not obtained with these measures, except possibly an occasional accidental occurrence, because a larger tolerance must be permitted for the machining of the threads to assure that the screw connection is possible. The symmetry plane of the double-cone nipple is therefore generally located above or below the contact surface of the electrode sections, with the result that the clearance in the two halves of the connection will be different. Since the contact resistance increases with the clearance and the Joule heat produced is proportional to the contact resistance, the nipple halves are heated up differently and large, different mechanical stresses are produced in the connection halves.
It is known to provide the symmetry plane with special markers, with the aid of which the symmetry plane can be brought into coincidence with the end faces of the electrode sections, and to fix the nipple in this position (German Published Prosecuted Application No. DE-AS 22 26 230). However, making this symmetrical connection is relatively cumbersome.